The present invention relates to a cell with a double liquid crystal layer using the electrically controlled birefringence effect and to a process for producing a uniaxial medium of negative optical anisotropy usable in said cell. It especially applies to the production of data display devices, such as matrix screens and in particular to the production of television screens.
Liquid crystal cells using the electrically controlled birefringence effect are already known. This effect has already made it possible to produce liquid crystal matrix screens and this is described in publications such as the article by J. ROBERT entitled "T.V. Image with L.C.D.", published in IEEE Transactions of Electron Devices, vol. Ed. 26, no. 8, August 1979 and the article by J.F. CLERC entitled "Electroopitical Limits of the E.C.B. Effect in Nematic Liquid Crystal", published in the journal Displays, October 1981.
A liquid crystal cell using the electrically controlled birefringence effect e.g. comprises, according to the state of the art, a nematic liquid crystal layer between two glass plates provided with transparent electrodes. Two polarizing means, e.g. two crossed rectilinear polarizers are respectively arranged on either side of the thus obtained assembly. When no voltage is applied between the electrodes, the molecules of the crystal layer are substantially parallel to a direction which is called the homeotropic direction" and perpendicular to the glass plates and incident light cannot pass through the cell. When an appropriate voltage is applied between the electrodes, the molecules of the liquid crystal layer are oriented substantially in a direction which, with the homeotropy direction, forms an angle which is a function of the voltage applied or excitation voltage. Incident light can then at least partly traverse the cell. It is therefore possible to electrically control the light intensity transmitted through said cell, said intensity being a function of said voltage.
The liquid crystal cells using the electrically controlled birefringence effect suffer from the disadvantage that the contrast of these cells, when observed obliquely, deteriorates and this increases with the observation angle and the contrast can even be reversed for certain observation angles.
French patent application No. 8407767 of May 18, 1984 proposes a liquid crystal cell using the electrically controlled birefringence effect aiming at obviating this disadvantage.
Cells of the type described in the aforementioned application can be produced with significant liquid crystal thicknesses as required for the production of complex screens (having a large number of picture elements).
However, the use of large liquid crystal thicknesses for producing the cell leads to slow molecular movements within the cell and therefore to slow optical response times on the part thereof. These times are only compatible with a rate of approximately 10 pictures per second and are consequently incompatible with an application of said cell to the production of television screens.
Moreover, the use of large liquid crystal thicknesses leads to parasitic visual effects around the image points of such a cell, the contours of said points being "white", even when the cell is in the "black" state (corresponding to a voltage between the cell electrodes below a threshold voltage).